1. Field of the Invention
The invention relates to the steam reforming of fluid hydrocarbons. More particularly, it relates to an improved process and apparatus for reducing the fuel consumption of such steam reforming operations.
2. Description of the Prior Art
In the primary steam reforming of fluid hydrocarbons, such as natural gas, the feed material and steam are passed through catalyst-containing vertically hanging reformer tubes maintained at an elevated temperature by radiant heat transfer and/or by contact with combustion gases in the furnace portion of the tubular reactor. The hot reformer tube effluent may be passed to a waste heat recovery zone for the generation of steam that can be used in the steam reforming operations. Conventional primary steam reforming operations are commonly carried out at temperatures of from about 750.degree. C. to about 850.degree. C. or above, with a mole ratio of steam to hydrocarbon feed of about 2/1-4/1.
The primary steam reforming reaction is a highly endothermic reaction, and the large amounts of required heat are typically provided by combusting external fuel at close to atmospheric presures in the reforming furnace. The walls of the reformer tubes must necessarily be capable of withstanding extreme operating conditions, such as skin temperatures on the order of 750.degree.-880.degree. C. and pressure differences of about 15-40 bars. Consequently, the reformer tubes are generally made of high alloy, expensive materials having a limited operating life under such extreme conditions. The reaction temperatures existing inside the reformer tubes are generally lower than about 850.degree. C. so that the effluent gas recovered from the primary reformer typically contains 2-6% methane.
In further accordance with conventional practice, the effluent from primary reforming is sometimes passed to a secondary reforming zone in which unconverted methane present in the reformed gas mixture is catalytically reacted with air, oxygen or other suitable oxygen-containing gas. The secondary reforming reaction of methane and oxygen is an exothermic combustion reaction in which the temperature rises generally to above 950.degree. C., with no external heat being supplied as in primary reforming. The walls of the secondary reforming reactor can thus be protected by refractories and kept at much lower temperatures, e.g. 300.degree. C., than is the case with the primary reformer tubes. Instead of such reactor tubes, a single, large diameter secondary reforming reactor can be employed using less costly materials than must be employed in the primary reformer. Because of the very high reaction temperature employed, very little unconverted methane remains in the effluent gas removed from the secondary reformer reactor.
Large quantities of hydrogen, or of an ammonia syngas mixture of hydrogen and nitrogen, are produced either by such steam reforming operations or by partial oxidation reactions. Partial oxidation, like secondary reforming, is an exothermic, autothermal, internal combustion process. While secondary reforming is also a catalytic process, however, the various known partial oxidation processes employ non-catalytic reactions, and thus operate at higher reaction temperatures on the order of about 1300.degree. C. The significant advantages obtainable by use of secondary reforming, or by the use of partial oxidation processing, are off-set to some extent by the need to compress the oxygen-containing gas to the desired reaction pressure or higher. Another disadvantage of secondary reforming and of partial oxidation processing is that part of the feed gas is combusted to carbon dioxide and water instead of to desired product. As a result, more natural gas or other feed gas is required to produce a given amount of hydrogen or synthesis gas, although the autothermic processes do not require any fuel. By contrast, the fuel consumption rate for primary reforming is typically between 30% and 50% of the feed rate.
Those skilled in the art will appreciate that it is not practical to employ secondary reforming processes alone, apart from an initial primary reforming of the feed gas. In practical commercial operations, therefore, primary reforming alone or partial oxidation with oxygen are the most frequently employed processes for the production of pure hydrogen product. When it is desired to produce an ammonia syngas mixture of hydrogen and nitrogen, on the other hand, a combination of primary reforming followed by secondary reforming, with air rather than oxygen, is most commonly employed. While such a combination of primary and secondary reforming is partly autothermic, in that no external fuel requirement exists for the secondary reformer, it nevertheless has the disadvantages of requiring the use of a relatively large primary reformer and of having relatively low thermal efficiency. Such disadvantages have been recognized in the art and efforts have been made to improve the overall process by the recovery of heat in order to reduce the size of the external fuel-fired primary reformer furnace. In the Quartulli et al, U.S. Pat. No. 3,264,066, the problems peculiar to primary-secondary reforming operations for the production of ammonia synthesis gas were addressed, including the requirements for large sized equipment and for the use of large amounts of steam and fuel under desirable operating conditions. Quartulli et al disclose the use of a heat exchanger between the primary and secondary reformers for indirect heat exchange of the primary and secondary reformer effluents. The temperature of the primary reformer effluent, which is the feed to the secondary reformer, is thereby raised, while the temperature of the effluent from the secondary reformer is decreased. In the Crawford et al, U.S. Pat. No. 4,079,017, another approach is suggested wherein parallel steam reformers are used for the primary reforming of a hydrocarbon feed. One portion of the feed is heated by means of radiant heat, i.e. by use of a steam reforming furnace, while another portion is heated by indirect heat exchange with the effluent from the secondary reforming operation, i.e. in an exchanger-reactor unit. While the approach of both of these patents is to recover heat for utilization in the reforming reactions, thus reducing the size of the external fuel-fired primary reformer, either all of the feed, as in U.S. Pat. No. 3,264,066, or at least a major portion thereof, as in U.S. Pat. No. 4,079,017, passes through such a primary reformer. Both patents also have the disadvantage of the typical apparatus problems that are commonly encountered due to the difficult mechanical design problems associated with conventional-type heat exchangers operated at the relatively high temperatures involved in the reforming application.
Another approach to improving steam reforming operations by reduction of fuel consumption is disclosed in the Fuderer, U.S. Pat. No. 4,337,170. This patent teaches the reforming of 20-30% of a feed stream in a primary reformer-exchanger unit in which the hot product effluent from conventional reforming, together with the hot product effluent from the reformer-exchanger itself, supplies the heat for said reformer-exchanger unit. The conventional reforming comprises either conventional primary reforming alone, or such primary reforming coupled with a secondary reforming operation. In the latter case, the hot effluent from the secondary reformer passes to the reformer-exchanger. By contrast with the approach of Crawford et al wherein the product effluent of each of the parallel primary steam reformers is necessarily passed to a secondary reformer with the product effluent therefrom being used to supply the heat required for the primary reforming of a portion of the feed stream, the Fuderer approach does not require the use of a secondary reformer. While the processing flexibility afforded thereby is desirable, the portion of the feed stream that passes directly to the reformer is not subjected to secondary reforming in any event, even when a secondary reformer is used to treat the effluent from a conventional primary reformer. As a result, the residual methane concentration of the mixed product effluent is much higher than that of a product stream from secondary reforming. This loss of unconverted methane is not desirable even though the use of a reformer-exchanger as disclosed by Fuderer enables a significant reduction in fuel consumption to be achieved together with other operating advantages. As with the techniques of Quartulli et al and Crawford et al, it also will be seen that Fuderer requires that an external fuel-fired primary reformer furnace be employed, although the fuel requirements thereof are reduced.
Despite such efforts to improve steam reforming operations, it will be appreciated that there remains a desire in the art to achieve lower steam and fuel requirements and higher thermal efficiencies in such operations. In addition, improved mechanical designs are also desired to reduce the size of the overall reforming systems employed and to achieve other useful purposes, such as a reduction for the thermal stresses to which the primary reformer tubes are subjected. It is also desired to carry out steam reforming operations at higher pressures, as in the range of 20-100 Bar. Such desired improvements also relate to the integration of primary and secondary reforming operations, so as to obtain the benefits of secondary reforming while achieving a more efficient overall reforming operation than has heretofore been possible in the art.
It is an object of the invention, therefore, to provide an improved process and apparatus for the reforming of hydrocarbons.
It is another object of the invention to provide a process and apparatus for minimizing the fuel requirements of reforming operations.
It is another object of the invention to provide a process and apparatus for the integrated primary and secondary reforming of hydrocarbons.
It is another object of the invention to provide a reforming process having low steam requirements and enhanced thermal efficiency.
It is a further object of the invention to provide a primary and secondary reforming apparatus of compact design and of reduced thermal stress of the primary reformer tubes.
It is a further object of the invention to provide a process and apparatus for carrying out steam reforming operations at higher pressures, as in the range of about 20-100 Bar.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.